1. Field of the Invention
The present invention relates to improved cement compositions and methods of using the compositions in cementing oil and gas wells.
2. Description of the Prior Art
In cementing operations carried out in oil, gas and other wells, a hydraulic cement and various additives are mixed with sufficient water to form a pumpable slurry, and the slurry is pumped into a subterranean zone to be cemented by way of the well bore penetrating the zone. After placement, the cement slurry sets into a hard mass.
In primary cementing whereby casing and/or one or more liners are bonded within the well bore, the cementing is accomplished by introducing a cement composition into the annular space between the casing and/or liners and the well bore. Generally, the cement composition is pumped downwardly within the casing and/or liners to the bottom thereof and then upwardly into the annulus.
In wells which are drilled into or through high pressure formations, it is necessary that the cement compositions used in carrying out cementing operations in the formations have high densities in order to provide high hydrostatic pressure and to thereby prevent blow-outs In wells which are drilled into or through low pressure formations, it is generally inadvisable to expose the formations to high hydrostatic pressures in that the formations may break down and the cement compositions lost. Thus, in cementing wells drilled into and through low pressure formations, lightweight cement compositions are advantageously utilized.
In most wells, one or more of the subterranean formations penetrated by the well bore contain gas under pressure. When primary cementing operations are carried out in the annulus, the gas may penetrate the cement composition whereby gas migration occurs. Gas migration generally starts during the setting process of the cement composition, i.e., during the time the cement composition changes from a hydrostatic fluid to a solid state body. If the cement composition is not able to resist the gas pressure during the setting process, channels can be formed in the partially cured composition whereby gas flows through the column between formations and/or to the surface.
In order to produce both heavyweight and lightweight cement compositions having improved properties whereby the compositions resist gas migration, have low strength retrogression at high temperatures and do not readily settle or segregate, finely divided amorphous silica dust has heretofore been included in the cement compositions. For example, U.S. Pat. No. 32,472 issued on Sep. 6, 1988 discloses a lightweight well cement composition in which amorphous silica dust obtained during the electrothermal preparation of ferrosilicon or the like is included therein. Such silica dust particles typically have specific surface areas of from about 18 to about 22 square meters per gram (m.sup.2 /g). Upon and during hardening, the cement composition is substantially gas tight (prevents gas migration) and provides high compressive strength.
U.S. Pat. No. 4,935,060 issued Jun. 19, 1990 discloses a high density hydraulic cement composition which is gas tight, has a very low strength retrogression at high temperature and has substantially no tendency to settle or segregate. The cement composition contains amorphous silica dust particles which have a specific surface area in the range of from about 15 m.sup.2 /g to about 30 m.sup.2 /g.
While the foregoing and other prior art cement compositions have achieved varying degrees of success, there is a need for improved cement compositions having both low and high densities which have reduced settling tendencies, better strengths, enhanced fluid loss properties and which prevent gas migration.